GSP being stonger and having a good posture at the time.. and sweat.. would have nothing to do with it? And I see where the shoulders could be an issue, but the video I saw had baby jay in the upper ribs area.

Bj has been able to do it with bigger / stronger and sweaty guys before...
I am sure it has an effect, but come on, his legs were slipping off like crazy.

I can't find the link in the forums, but somebody posted a thread with a link to a GSP / greased blog website, with a ton of GIF's of Penn trying to get into the rubber guard and he just could not, his flexibility might be his biggest weapon.